Waitaki District Council communications and policy officer Scott William Ridley and Christchurch truck driver Anthony (Tony) Lyn Smith died in the crash, which occurred in heavy fog about 4.50am on March 31.
Mr Ridley (23) was returning to Oamaru from Dunedin, where he was studying part time at the University of Otago, and Mr Smith (65) was driving south.
Otago-Southland coroner David Crerar found Mr Ridley crossed the centre line and collided with Mr Smith's truck. Mr Smith died in the crash and Mr Ridley died later in Dunedin Hospital.
Mr Crerar, while not adamant about the cause, believed Mr Ridley was affected by fatigue and might have gone to sleep, even briefly.
Mr Ridley's mother, Trish Le Sueur, said the reason her son crossed the centre line might never be known.
She said her son was "very driven", performing well in his work with the council and chose to live his life at a pace most could not match.
She was "truly sorry" for the pain caused to those affected and said her family would live with it forever.
Two Dunedin friends of Mr Ridley gave evidence that between about 6pm, after a meal, and midnight he drank six stubbies of beer. After that he drank nothing, telling them he intended to drive back to Oamaru that night.
Justin James Boswell said, before Mr Ridley left about 3.30am, they jokingly did a breath test, which came up green, using a "gimmick breathalyser".
Neither of the two witnesses regarded him as being intoxicated when he left.
Serious crash unit investigator Senior Constable Trevor Buchanan said the fog was not considered a factor, based on evidence from a truck driver who saw the crash.
He found it was caused by Mr Ridley driving while fatigued and, because of a lapse of attention on Mr Ridley's part or because he fell asleep, veering across the centre line into the path of the truck.
Toxicology reports showed Mr Ridley had a nil alcohol reading in his blood, although Sergeant Tony Woodbridge, of Oamaru, said that might not have been the case at the time of the crash because Mr Ridley had received transfusions during treatment at the scene, in the rescue helicopter and at Dunedin Hospital.
Truck driver Jared Ross Buchanan, of Rangiora, who was following Mr Smith's truck south, estimated visibility in the thick low fog was between 30m and 50m, and said it was the worst fog he had driven in.
He said, as they approached the bridge over Baghdad Creek, just south of Hampden, he saw the lights of Mr Ridley's car about 150m away.
Just as it was on the bridge, the car veered across the road into the truck.
In an interim verbal finding, Mr Crerar believed, from the evidence, that Mr Ridley probably went to sleep, even briefly, which caused him to cross the centre line.
There was insufficient evidence to say Mr Ridley was intoxicated to the extent he should not have been driving and fatigue was a factor given he had been working, studying and socialising before driving.
However, the consumption of even a moderate amount of alcohol had a soporific effect, compounding the initial fatigue, he said.
Mr Ridley was probably tired and that, combined with a moderate amount of alcohol, had caused him to lose control of the car, he said.
He found Mr Ridley died from a cerebral injury in the crash, which occurred after he lost control of his car and crossed to the other side of the road, colliding with the truck.
He found Mr Smith died from a cerebral injury caused by blunt trauma received when the truck he was driving was struck by Mr Ridley's car.
